The Mycobacterium tuberculosis health crisis is exacerbated by the alarming emergence of multi-drug resistant strains. The development of new chemotherapeutic strategies is imperative, which requires insight into the pathways involved in M. tuberculosis infection and drug resistance. Studies suggest that M. tuberculosis utilizes DNA repair to resist killing by genotoxins as well as to acquire antibiotic resistance. DNA microarrays in Mycobacterium smegmatis have implicated two proteins to be involved in regulating DNA repair: the CarD-like transcriptional regulator (CarD) and Arr ADP-ribosyl transferase (Arr). These unexplored genes stand out due to their >2 fold level of induction following DNA damage, presence in published gene sets from similar screens, and predicted functions in controlling DNA repair. Therefore, the objective of this study is to elucidate the functions of CarD and ADP-ribosylation during mycobacterial DNA damage by integrating genetics, biochemistry, and in vivo disease modeling into three specific aims. First, the roles of CarD and Arr during DNA repair will be established by analysis of M. smegmatis null mutants in genotoxic killing assays. Second, after investigating the roles of CarD and Arr during DNA damage, their respective targets will be identified. DNA microarrays will be used to gain insight into the downstream factors in CarD and Arr associated pathways. CarD transcriptional targets will be verified by in vitro DNA binding assays, while Arr post-translational modification targets will be investigated via in vitro ADP-ribosylation assays. Lastly, the functions of CarD and ADP-ribosylation during DNA damage in vitro will be confirmed in vivo by analyzing M. tuberculosis null mutants in the mouse model of infection and in cultured macrophages. These investigations will provide critical insight into the mycobacterial DNA damage response and aid in future treatments of bacterial pathogens. Mycobacteria infections have an enormous impact on world health and the objectives of this application coincide with the mission of the NIAID to better understand, treat, and ultimately prevent mycobacterial infection. Mycobacterium tuberculosis results in approximately 8 million new cases of active tuberculosis and over 2 million deaths annually. The experiments proposed will provide critical insight into the evolution of antibiotic resistance and the pathogen's ability to resist host derived DNA damage to aid in future therapies of mycobacterial disease.